1. Field of the Invention
The present invention relates to a method and device for a wireless communication, and more particularly, to a method and device capable of ensuring operations of a channel estimation of a very high throughput (VHT) wireless local area network (WLAN).
2. Description of the Prior Art
Wireless local area network (WLAN) technology is one of popular wireless communication technologies, which is developed for military use in the beginning and is widely applied in consumer electronics in recent years, e.g. desktop computers, laptop computers, personal digital assistants, etc., to provide convenient and high-speed internet communication media for the masses. IEEE 802.11 is a set of WLAN protocols developed by the Institute of Electrical and Electronics Engineers, evolved from the former IEEE 802.11a/b/g standards to the current mainstream IEEE 802.11n standard. IEEE 802.11a/g/n standards adopt orthogonal frequency division multiplexing (OFDM) technique. Different to IEEE 802.11a/g standard, IEEE 802.11n standard further applies a multiple-input multiple-output (MIMO) technique and other features, and greatly enhances data rate and throughput. In addition, in IEEE 802.11n standard, the channel bandwidth is doubled from 20 MHz to 40 MHz.
Please refer to FIG. 1, which is a schematic diagram of a conventional IEEE 802.11n packet. As shown in FIG. 1, the IEEE 802.11n packet consists of a preamble portion carrying preamble data and a payload portion carrying data to be transmitted. The preamble data is of a mixed format, is backward compatible with IEEE 802.11a/g standard devices, and includes, in sequence, legacy short training field L-STF, legacy long training field L-LTF, legacy signal field L-SIG, high-throughput signal field HT-SIG, high-throughput short training field HT-STF, and N pieces of high-throughput long training fields HT-LTF. The legacy short training field L-STF is used for start-of-packet detection, automatic gain control (AGC), initial frequency offset estimation, and initial time synchronization. The legacy long training field L-LTF is used for further fine frequency offset estimation and time synchronization. The legacy signal field L-SIG carries information of data rate and packet length. The high-throughput signal field HT-SIG also carries data rate information, and is used for packet detection, so that the mixed format or the legacy format the transmitted packet uses can be detected. The high-throughput short training field HT-STF is used for automatic gain control. The high-throughput long training fields HT-LTF are used for MIMO channel detection, enabling receivers to determine channel status accordingly.
Patterns of the high-throughput long training fields HT-LTF are well-known for those skilled in the art, and are not narrated herein. Functionally, the high-throughput long training fields HT-LTF can be further divided in two categories. The first category refers to data high-throughput long training fields, for estimating a channel status used by current data, with a quantity NDLTF determined by a quantity NSTS of space time streams, as illustrated in FIG. 2. The second catalog refers to extension high-throughput long training fields, for detecting extra spatial dimensions of channels not in use, with a quantity NELTF determined by a quantity NESS of extra spatial dimensions to be detected. A reference table of NELTF and NESS is identical to that of NDLTF and NSTS as illustrated in FIG. 2. In addition, IEEE 802.11n standard supports upmost four antennas, and hence, NDLTF and NELTF are smaller or equal to 4.
On the other hand, in order to reduce the complexity of channel estimation, the high-throughput long training fields HT-LTF are designed to be generated by giving weightings and delays to a single symbol in the prior art. Therefore, as illustrated in FIG. 3, aimed at different transmission routes (TX1-TX4), a transmission terminal passes the high-throughput long training fields HT-LTF through a spreading code matrix, to determine adequate weightings in channel estimation, performs a cyclic shift delay (CSD) execution to add cyclic prefixes for resisting multi-path interference, performs a spatial mapping execution, such as beamforming, for enhancing SNR, and finally, performs an inverse discrete Fourier transform to realize OFDM, transforming input sequences of frequency domain into OFDM symbol sequences of time domain. The spreading code matrix is a 4×4 matrix P4×4:
      P          4      ×      4        =            [                                    1                                              -              1                                            1                                1                                                1                                1                                              -              1                                            1                                                1                                1                                1                                              -              1                                                                          -              1                                            1                                1                                1                              ]        .  
To achieve WLAN transmission with much higher quality, the IEEE committee is developing new WLAN standards, such as IEEE 802.11ac standard, which has features of very high throughput (VHT), increased channel bandwidth from 40 MHz to 80 MHz, and supporting more than four antennas. In other words, the quantity NSTS of space time streams (or the quantity NESS of extra spatial dimensions to be detected) may exceed 4, i.e. exceed the situation defined in FIG. 2. As a result, the quantity NDLTF of data high-throughput long training fields (or the quantity NELTF of extension high-throughput long training fields) cannot be accurately decided, and the spreading code matrix cannot be decided at the same time.
Therefore, it is necessary to determine the quantity NDLTF of data high-throughput long training fields (or the quantity NELTF of extension high-throughput long training fields) when the quantity NSTS of space time streams (or the quantity NESS of extra spatial dimensions to be detected) is greater than 4, to facilitate the realization of the next generation WLAN standard.